Factors causing the population limitation of two species of cyclopoid copepods, Tropocyclops prasinus and Cyclops bicuspidatus thomasi, and the subsequent impact of these species on the zooplankton community, were investigated in two montane lakes of the University of British Columbia Research Forest. In Placid Lake, C.b.thomasi was abundant and T.prasinus was rare, while in Gwendoline Lake, T.prasinus was the more abundant species. Field and laboratory experiments were conducted to assess the importance of such factors as food, physical-chemical lake characteristics, and predation upon the population dynamics of these species. The feeding appendages of adult T.prasinus and C.b.thomasi were compared and found to be structurally alike, although adult diets were quite dissimilar. T.prasinus was omnivorous in the later instars, eating algae, protozoans and probably dead macrozooplankters while C.b.thomasi was markedly carnivorous in copepodid and adult instars. Seasonal abundance peaks revealed a temporal separation of the naupliar instars of these species in both lakes studied. Consequently, competition between T.prasinus and C.b.thomasi was unlikely. Large scale enclosure experiments in Placid Lake, where C.b.thomasi was abundant, showed that lake densities of C.b.thomasi could limit the abundance of T.prasinus. Survivorship curves and laboratory feeding experiments indicated that the low abundances of T.prasinus was caused by two important factors: high T.prasinus naupliar mortality even in the absence of C.b.thomasi, and C.b.thomasi predation on these T.prasinus instars. When C.b.thomasi was transplanted to enclosures in Gwendoline Lake, where T.prasinus was the more abundant cyclopoid copepod, C.b.thomasi was able to grow and reproduce within the crustacean plankton community and became as abundant as in its home lake. Naupliar instars were the most mortality-prone stages in both lakes. Fertilization of a Gwendoline Lake enclosure increased the survivorship of C.b.thomasi about 22% over that observed in the unfertilized treatment. However, addition of the midge fly larvae common to Gwendoline Lake (C.trivittatus and C.americanus) to the enclosure plankton community resulted in a decrease in the standing crop of C.b.thomasi to a point at which samples became difficult to obtain. With Chaoborus in the enclosure, fertilization did not enhance the abundance of C. b. thomasi but rather, improved the survivorship of Chaoborus first and second instar larvae, and resulted in an acceleration of the C.b.thomasi nauplii decline. The impact of C.b.thomasi on other zooplankton was also examined. C.b.thomasi appeared to dampen the fluctuations in total zooplankton biomass in enclosures without a Chaoborus population. However, in the fertilized treatment without Chaoborus, C.b.thomasi was unable to track prey demographic responses and caused no major compositional changes in the crustacean community at either high or low nutrient levels. Temporal - spatial overlap and predator and prey developmental responses were critical factors determining prey sensitivity to predation by C.b.thomasi.Science, Faculty ofZoology, Department ofGraduat
